1. Field of the Invention
This invention relates to a new powder lacquer based on combinations of organic polyhydroxyl compounds with organic polyisocyanates as crosslinking agent and to the use of this powder lacquer in the production of coatings on heat-resistant substrates.
2. Description of the Prior Art
Polyurethane-based powder lacquers are known (for example DE-AS 1,957,483, DE-OS 2,047,718, DE-OS 2,064,098, DE-PS 2,127,839, DE-OS 2,246,620, DE-AS 2,351,477, DE-OS 2,429,517). The hardener components used are blocked polyisocyanates. Hardeners prepared from isophorone diisocyanate (IPDI) and generally blocked with .epsilon.-caprolactam have often been described for light-stable lacquer systems (for example DE-OS No. 2,105,777, 2,542,191, 2,735,497, 2,842,641, 2,929,224, 3,004,876, 3,039,824).
Lacquers combining easy handling with good flow, high hardness and elasticity and good resistance to chemicals are obtained from these .epsilon.-caprolactam-blocked IPDI derivatives after baking with suitable polyhydroxyl compounds. Unfortunately, one disadvantage of these polyurethane-based powder lacquers is the high baking temperature of approximately 200.degree. C. Another disadvantage lies in the elimination of the blocking agent which means that special measures have to be taken during the processing of the powder lacquers to clean the exhaust air and to recover the blocking agent. The elimination of the blocking agent is also a barrier to the introduction of new techniques, including for example the use of direct, gas-heated baking ovens and the use of infrared hardening.
There has been no shortage of attempts to lower the high baking temperatures by using other blocking agents. For example, triazoles (DE-OS 2,812,252), cyclic amidines (DE-OS 2,946,085) and secondary amines (DE-OS 3,434,881) have been proposed as blocking agents with relatively low elimination temperatures for polyurethane powder lacquer hardeners.
However, this does not eliminate the fundamental disadvantage of the presence of blocking agents in the powder lacquer. The above-described difficulties which can arise through the eliminated blocking agent remain and preclude having a resin which completely reacts to a lacquer film, despite the absence of solvents.
An attempt to overcome this fundamental disadvantage was made by using unblocked, linear uretdione- and urethane-group-containing IPDI derivatives which contain terminal urethane, urea or free NCO groups (EP-A-0,045,994, EP-A-0,045,996 or EP-A-0,045,998). When crosslinking agents such as these are used, the polyhydroxyl compounds are crosslinked through the splitting up of the uretdione rings. The disadvantage here is the strict linearity of the crosslinking agent which is necessary for the synthesis process and which precludes branching of the lacquer to impart high solvent resistance, scratch resistance and extreme hardness by variation of the hardener.
Accordingly, an object of the present invention is to provide a new powder lacquer which is not attended by any of the above-mentioned disadvantages of the prior art, i.e. which in particular may be baked at low temperatures to form highly crosslinked lacquers without the elimination of blocking agents.
According to the invention, this object may be achieved with the powder lacquers described in detail hereinafter.
The hardener component of the powder lacquer according to the invention is based on a specific unblocked polyisocyanate or polyisocyanate mixture. The concept of using unblocked polyisocyanates having a high concentration of free isocyanate groups as hardeners was by no means obvious because a powder lacquer of the type in question containing free hydroxyl groups and free isocyanate groups would normally be expected to undergo premature crosslinking, thus leaving the ready-to-use lacquer with an inadequate shelf life.
Although the use of unblocked polyisocyanates as crosslinking agents for powder lacquers is proposed in EP-A-0,071,812, crosslinking takes place through an NCO/epoxide reaction according to the teaching of this publication. This reaction has to be carried out in the strict absence of free hydroxyl groups because otherwise crosslinking and gelation would occur during the mixing and extrusion of the components.
Although isocyanates containing urethane groups, of the type which may be used as component (A) in accordance with the present invention, are already known (EP-A-0,182,996), it is emphasized in this publication that combinations of these urethane-modified polyisocyanates with polyhydroxyl compounds represent two-component systems, i.e. systems which only have a limited pot life.
Although the polyisocyanates containing isocyanurate groups according to Applicants' earlier German Patent Application P 35 07 719.0 may also be used inter alia as unblocked crosslinking agents for powder lacquers, the teaching of this patent application does not belong to the published prior art.